1. Field of the Invention
The present invention relates generally to an apparatus for generating a variable compression ratio in an internal combustion engine, including an apparatus wherein an inner piston is selectively movable within an outer piston.
2. Description of the Related Art
In automotive powertrain designs that currently prevail, an internal combustion engine (ICE) is employed as the source of motive power. ICEs create mechanical work from fuel energy by combusting the fuel over a thermodynamic cycle. Although the demands of normal driving call for a wide range of power demands and speeds, the best energy conversion efficiency of an ICE is experienced over only a relatively narrow range of loads and speeds.
ICEs sized and calibrated to generate the high power levels required to meet intermittent demands (such as rapid acceleration, passing, and hill climbing) operate inefficiently at low to moderate power levels the vast majority of the time. This is largely because, with conventional technology, the compression ratio cannot be calibrated and is therefore pre-set to a level that will allow the ICE to meet intermittent power demands, as opposed to a level that will optimize engine efficiency during normal operating loads.
Compression ratio is the ratio of expanded cylinder volume to compressed cylinder volume in one cycle of a reciprocating piston within an ICE. According to thermodynamic laws, a greater degree of compression relative to the expanded volume corresponds to greater efficiency of the thermodynamic cycle and hence greater efficiency of the engine. An ICE with a higher compression ratio is therefore better able to convert fuel energy to mechanical work than an ICE with a lower compression ratio. Unfortunately, a high compression ratio may result in several undesirable side effects. An increased level of friction and higher peak cylinder pressures are two results of a high compression ratio. Under these conditions, if the fuel is introduced with a fresh charge of air, there is a potential for knocking or pre-ignition at high power output.
For this reason, with conventional engine hardware, if the compression ratio were simply pre-set to a high level in order to maximize engine efficiency at normal loads, the operation of the ICE at the maximum power demand levels would lead to severe knocking, reduced engine efficiency, and potential engine damage.
These problems could be avoided if the compression ratio of an ICE could be calibrated. Ideally, one would desire to employ a high compression ratio at normal loads, and shift to a lower compression ratio for intermittent high loads. In this way, the high efficiency associated with a high compression ratio could be achieved over normal ranges of operation, while higher power output could be achieved without fear of pre-ignition by invoking a lower compression ratio.
Various methods are currently known to vary the compression ratio of an ICE. However, as testified to by the lack of variable compression ratio engines in automotive applications, none of these known designs have proven to be sufficiently effective or practical to warrant widespread use in automotive applications. Applicant therefore believes it is desirable and possible to provide an improved system for generating a variable compression ratio engine. The present invention provides such a system.
Briefly, the present invention provides an improved system for generating a variable compression ratio within an ICE. The engine may therefore operate at more than one distinct compression ratio, selectable during engine operation. As a result, an engine provided in accordance with the present invention operates near its most efficient operating range during the majority of driving, while providing intermittent high power capability in a way that does not lead to undesirable side effects. (While the invention is described herein as used in an automotive ICE, it will be understood that the present invention may be used in any ICE.)
More particularly, in a preferred embodiment of the present invention, a piston assembly for an ICE has an inner piston slidably mounted within an outer piston. The outer piston is mounted in a cylinder of an ICE to reciprocate in a conventional manner. During operating conditions of low to moderate power demands, the top of the inner piston is flush with the top of the outer piston, defining a high compression ratio mode. The relatively high compression ratio in this mode provides improved thermodynamic efficiency in this operating range. When power demand increases to the point where this high compression ratio might cause performance problems such as pre-ignition or knocking, a command signal causes the inner piston to recede to a second position within the outer piston, thereby reducing the compression ratio. Good mixing and combustion is retained in both modes because the piston bowl resides within the receding inner piston and therefore does not change shape, only changing its relative distance from the top of the cylinder when at top dead center (TDC).
In a preferred embodiment, the inner piston is located in either the normal high compression ratio position or the intermittent low compression ratio position by the rotation of a rotary cam-like actuator which pivots about a wrist pin residing in the outer piston. (It will be understood that while the present invention has been described in the context of an application where a higher compression ratio is the predominant mode of operation and a low compression ratio is only used intermittently, the present invention may provide an engine where the default mode of operation is at a low compression ratio and a high compression ratio is used intermittently.) In one preferred embodiment, the actuator is comprised of a rotary hydraulic piston within a hydraulic chamber that is integrated with the wrist pin, and a cam which pivots around the wrist pin in reaction to movement of the hydraulic piston. Movement of the rotary hydraulic piston and cam assembly is caused by the presence or absence of pressurized fluid in the hydraulic chamber, in conjunction with inertial forces created by reciprocation of the piston assembly in an engine cylinder. The pressurized fluid is directed into and out of the hydraulic chamber by a control system that generates appropriate command signals. Additional embodiments vary the actuation means to include additional springs and/or hydraulic systems.